As a substrate for a semiconductor device or a solar battery, in general, a silicon substrate is used. As a material of such a silicon substrate, polycrystalline silicon manufactured by a Siemens method is used. However, as requirements for high integration and high quality of a final product become stricter, a high purity requirement for the polycrystalline silicon is also becoming stricter.
As light element impurities in silicon crystal, interstitial oxygen and substitutional carbon are known. The interstitial oxygen is an impurity that precipitates in crystal and causes dislocation and stacking fault. The substitutional carbon is an impurity that facilitates the precipitation of the oxygen. Concentration measuring methods for the interstitial oxygen and the substitutional carbon have been examined for a long time. For single crystal silicon, a standard measurement method employing Fourier-transform infrared spectroscopy has been established by agencies such as ASTM and JEIDA.
Incidentally, a large number of graphite members are used in a reactor used in precipitating polycrystalline silicon. Therefore, carbon generated from the graphite members tends to be taken into the polycrystalline silicon. Since such carbon impurities result in being taken into single crystal silicon manufactured from the polycrystalline silicon unless special removal of the carbon impurities is performed, improvement of purity of the single crystal silicon is hindered. Therefore, in order to realize low carbon concentration of the substrate for the semiconductor device or the solar battery, it is necessary to manage the carbon concentration of the polycrystalline silicon from which the substrate is manufactured.
Therefore, methods for evaluating carbon concentration in polycrystalline silicon have been examined and standardized. For example, one of ASTM standards (Non-Patent Literature 1) specifies a method for measuring carbon concentration in polycrystalline silicon according to a combination of a floating zone (FZ) method and an optical method (an infrared spectroscopy or a photoluminescence method).
In this method, first, a hole is drilled in a polycrystalline silicon rod obtained by being precipitated on a silicon core wire and a cylinder (a core) having a diameter of about 20 mm is pulled out. Subsequently, in order to eliminate damage caused on the core surface when the core is sliced out, the core surface is etched 100 μm or more by HNO3/HF mixed acid. A single crystal silicon rod is obtained by the FZ method using the core. For example, carbon concentration measurement is performed by the infrared spectroscopy conforming to Non-Patent Literature 2.
However, in the case of this method, because an effective segregation coefficient of carbon impurities in silicon crystal is small (keff=0.07: see Non-Patent Literature 3), there is a problem in that, if a single crystal rod grown by the FZ method is short, accurately measurement of carbon concentration in material polycrystalline silicon is difficult. More specifically, in the case of impurities having an effective segregation coefficient smaller than 1, the impurities included in the material polycrystalline silicon are concentrated on the material side because the impurities tend to be taken into an FZ melting region. Therefore, when it is attempted to accurately measure the carbon concentration of the material polycrystalline silicon, it is necessary to grow a long FZ single crystal rod.
Japanese Patent Application Laid-Open Publication No. 2007-279042 (Patent Literature 1) discloses a measuring method that makes use of the fact that carbon concentration in a freezing and melting region is higher than carbon concentration in a polycrystalline silicon composition. In this method, a polycrystalline silicon core extracted from the polycrystalline silicon composition is FZ-grown by an FZ crystal growing device to obtain a core including a single crystal region and a freezing and melting region. The core is annealed for at least two hours at temperature in a range of 1150° C. to 1360° C. Samples extracted from each of the single crystal region and the freezing and melting region are subjected to an infrared spectroscopic analysis to create a calibration curve and calculate the carbon concentration of the single crystal region. The carbon concentration of the polycrystalline silicon composition is determined based on the carbon concentration of the single crystal region.